Selector mechanism for a motor vehicle transmission

ABSTRACT

A rotatable selector mechanism for a motor vehicle transmission has a selector knob rotatable in a housing to select an operating mode of the transmission among a number of selector positions indicated by a label (P,R,N,D,M). The selector mechanism further includes a detent plate which cooperates with a detent pin for maintaining the selector knob in each of positions. The detent plate is connected to position sensors for generating selector signals to a transmission control unit which supplies control signals to an actuator for operate the transmission according to the operating mode selected. The detent plate is also associated to the selector knob and to a drive correction means by means of clutch devices respectively to engage the detent plate to one of the selector knob and the drive correction means. The clutch devices and the drive corrections means are controlled by the transmission control unit to move the detent plate from a selector position selected by the driver to another selector position representative of the operating mode of the transmission by means of the drive correction means without moving the selector knob. The selector knob further includes an illuminated pointer constituted by one of several LEDs controlled by the transmission control unit so as to change the LED illuminated in front of the label according to the position of the detent plate when the latter has been moved by the drive correction means in order to indicate the operating mode of the transmission.

FIELD OF THE INVENTION

The present invention relates to a selector mechanism for a motorvehicle transmission, more specifically an automatic or semi-automaticvehicle transmission actuated by shift-by-wire.

TECHNICAL BACKGROUND

A selector mechanism generally comprise a selector connected to positionsensors so as to select a selector mode, for instance Park (P), Reverse(R), Neutral (N) or Drive (D). The position sensors are connected to atransmission control unit supplying control signals to an actuator suchas a solenoid and a motor in order to drive a transmission in accordancewith output signals from the position sensors. Such vehicletransmissions when operated by shift-by-wire commonly include a widevariety of sensors, switches, and other controls. In the event that anyof these components fails, the transmission may fail to function. Ifsuch a failure is detected, the transmission control unit switches to afailure or ‘limp home’ mode which is designed to ensure that, as far aspossible, the vehicle will be drivable and safe. Generally in this modethe transmission control will attempt to maintain the current gear andthe failure is indicated to the driver by means of a warning light ortext message.

Despite the various warning alerting the driver that the transmission isin limp-home mode, it is still possible for the selector to be moved toa position to select an inappropriate selector mode, possibly causingimmediate and unexpected motion of the vehicle.

According to the particular type of selector mechanism, severalalternatives have been proposed to overcome this problem. Onealternative is disclosed on the Aston Martin™ DB9™ where the selectortakes the form of individual switches on the dashboard, each forselecting an operating mode for the transmission. In this alternative,the transmission control unit stops the engine if the driver selects anoperating mode which is not available. In the other forms of selector,i.e. a lever or a rotary knob, the driver must move the selector througha Neutral position to get a Drive position or a Reverse position. Insuch cases, if the transmission is in limp-home mode and the drivermoves the selector towards a mode which is unavailable the transmissioncontrol unit will shift the transmission to Neutral. Both of these knowntypes of alternative have certain inherent disadvantages for driverconvenience and safety.

The present invention has as an object the provision of an improvedselector mechanism for a vehicle transmission having an improvedinteraction with the driver when the transmission is in a limp-homemode.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided aselector mechanism for a motor vehicle transmission having a number ofoperating modes, the selector mechanism having a housing, a rotaryhandle movable in the housing between a number of selector positions,each position being associated with a respective one of the operatingmodes of the transmission, a rotary selector which in normal use of theselector mechanism is connected to the rotary handle so as to rotatewith the rotary handle, sensor means for sensing the position of therotary selector and generating selector signals to a transmissioncontrol unit which in use supplies control signals to the transmission,an indexing means for maintaining the rotary handle in each of itspositions, a drive correction means and clutch devices to engage therotary selector to one of the rotary handle and the drive correctionmeans, wherein the clutch devices and the drive correction means in useare controlled by the transmission control unit to move the rotaryselector from a selector position selected by the driver by means of therotary handle to another selector position representative of theoperating mode of the transmission by means of the drive correctionmeans without moving the rotary handle.

Preferably, the selector mechanism further comprises a dynamic indicatorwhich in use is arranged to indicate the operating mode of thetransmission. The dynamic indicator in use is connected to thetransmission control unit to be visible when the vehicle is to be drivenand move with the rotary selector.

Conveniently, the dynamic indicator comprises an illuminated pointer andan arrangement of illuminated labels indicating the selector mode. Theilluminated pointer may be constituted by a one of several light sourcescircumferentially distributed behind the top surface of the rotaryhandle.

The selector mechanism may further comprise a retractor actuator whichin use is controlled by the transmission control unit to displace therotary handle between an active position where it can be manually movedbetween the selector positions by the driver of the vehicle, and aninactive position in which it is retracted inside the housing where itcannot be readily moved by the driver.

Preferably, the selector mechanism comprises a spring-loaded detentmember carried by the housing and the rotary selector comprises a detentplate having notches for cooperation with the spring-loaded detentmember. The detent plate may have a slot which cooperates with a stopcarried by the housing to limit the rotation of the detent plate betweenthe two extreme selectable modes of the transmission.

Also preferably, the selector mechanism comprises a locking device whichin use is controlled by the transmission control unit to preventrotation of the rotary handle when disconnected from the rotaryselector.

Preferably one of the selector positions is a manual mode position inwhich the selector mechanism is operable for use with an auxiliaryselector in order to control the gear ratios of the transmission in apulse manner.

The invention also provides, according to a second aspect thereof, aselector mechanism for a vehicle transmission driven by an engine, thesaid selector mechanism having a housing, a selector movable in thehousing between a number of selector positions each associated with arespective operating mode of the transmission, sensors for sensing theposition of the selector and generating selector signals to atransmission control unit which in use supplies control signals to thetransmission, and a retractor actuator which in use is controlled by thetransmission control unit to displace the selector between an activeposition where it can be manually moved between the selector position bythe driver of the vehicle, and an inactive position in which it isretracted inside the housing where it cannot be readily moved by thedriver.

Conveniently, the selector is a rotary switch.

Preferably, the selector mechanism further comprising a drivecorrections means which in use is connected to the transmission controlunit to move the selector from a selector position selected by thedriver to another selector position representative of the operating modeof the transmission.

Preferably, the drive means is operable to prevent manual movement ofthe selector according to the state of the vehicle, i.e. in Parkposition.

The invention also provides, according to third aspect thereof, atransmission control system for a motor vehicle transmission having atransmission control unit which in use is connected to a communicationport at which data relating to the vehicle's operation are available andincluding a selector mechanism which is in accordance with said first orsecond aspect.

The invention also provides, according to a fourth aspect thereof, amethod of controlling a rotatable selector mechanism according to saidfirst aspect when installed in a motor vehicle and the transmissioncontrol unit is in receipt of a data relating to the operational stateof the vehicle in which the transmission control unit determines, afterreceiving a signal of the sensors, whether the selector mode isavailable and if the selector mode is unavailable the transmissioncontrol unit operates the clutch devices to engage the rotary selectorto the drive correction means and disengage the rotary selector from therotary handle, then send a command to the drive correction means toreturn the rotary selector to the previous gear position.

Also preferably, when the selector mechanism includes a dynamicindicator, the method includes sending a command from the transmissioncontrol unit to the dynamic indicator to indicate the mode of thetransmission according to the motion of the rotary selector.

Preferably, when the selector mechanism includes an illuminated pointerhaving several light sources and an arrangement of illuminated labels,the transmission control unit sends a command to turn the illuminatedpointer off and a command to illuminate the label of the currentselector mode.

Also preferably, when the selector mechanism includes a retractoractuator, the method includes sending a command from the transmissioncontrol unit to the retractor actuator to move the rotary switch to theactive position when the engine is running and to move the rotary switchto the inactive position when the engine is stopped.

Conveniently, if the transmission control unit detects or receives afailure signal, the transmission control unit sends a command to theretractor actuator to move the selector towards the inactive positionfrom the active position.

Preferably, when the selector mechanism includes a locking device, ifthe transmission control unit detects or receives a failure signal, thetransmission control unit sends a command to the locking device toprevent rotational movement of the rotary handle.

Conveniently when the selector mechanism includes an auxiliary selectorand if the transmission control unit detects or receives a signal fromthe auxiliary selector, the transmission control unit sends a command tothe drive correction means and the dynamic indicator so as to rotate therotary selector and change the indication of the dynamic indicator fromindicating a Drive mode to indicating a Manual mode. Similarly, if theselector mechanism is in a manual mode and the transmission control doesnot detect or receive a signal from the auxiliary selector for apredetermined time period, the transmission control unit may send acommand to the drive correction means and the dynamic indicator so as torotate the rotary selector and change the indication of the dynamicindicator from the Manual mode to the Drive mode.

The invention also provides, according to a fifth aspect thereof, amethod of controlling a selector mechanism according to said secondaspect when installed in a motor vehicle, and the transmission controlunit is in receipt of a data relating to the operational state of thevehicle, in which, if the transmission control unit detects or receivesa failure signal, the transmission control unit sends a command to theretractor actuator to move the selector towards the inactive positionfrom the active position.

Preferably, the method includes sending a command from the transmissioncontrol unit to the retractor actuator to move the selector to theactive position when the engine is running and to move the selector tothe inactive position when the engine is stopped.

Also preferably, when the selector mechanism includes a drive correctionmeans, the transmission control unit determines after receiving a signalof the sensors, whether the selector mode is available and if theselector mode is unavailable the transmission control sends a command tothe drive means to return the selector to the previous gear position.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example with reference tothe accompanying drawings, of which:

FIG. 1 is a plan view showing a selector mechanism for a vehicletransmission according to the present invention;

FIG. 2 is a perspective view of the selector mechanism shown in FIG. 1;

FIG. 3 is an exploded view of the selector mechanism shown in FIG. 1;

FIG. 4 is a cross-section on the line IV-IV of FIG. 2, the selectormechanism being shown in normal operation;

FIG. 5 is a view similar to FIG. 4 but with the selector mechanism in acorrection operation,

FIG. 6 is a plan view of the selector mechanism shown in FIG. 3 but withsome parts omitted to show certain components in a Park position;

FIG. 7 is view similar to FIG. 6 showing the components in a Driveposition;

FIG. 8 is a block diagram showing a transmission control system of amotor vehicle equipped with a selector mechanism according to thepresent invention.

FIG. 9 is perspective view showing a modification to the selectormechanism shown in FIGS. 1 to 8, the selector mechanism being located ina console between the front seats of a vehicle passenger compartment andhaving a selector shown in an active position; and

FIG. 10 is a view based on FIG. 9 showing the selector in an inactiveposition,

DETAILED DESCRIPTION

With reference to the drawings, there is shown a rotatable selectormechanism 10 for actuating an automatic vehicle transmission 11 drivenby an engine 12. The rotatable selector mechanism 10 is located in anupper surface of a console mounted between a driver's seat and apassenger seat.

The selector mechanism 10 includes a housing formed by a base 13 and apart 14 of the console inside which a rotary handle in the form of aselector knob 15 is rotatable around a substantially vertical fixedshaft 17 mounted to the base 13. The selector knob 15 can be grasped bythe driver and rotated around the shaft 17 to select the operating modeof the transmission by pointing an illuminated pointer 16 towards one ofa number of labels constituted by the letters PRND. These letterscorrespond to the Park (P), Reverse (R), Neutral (N) and Drive (D) modesrespectively in the automatic transmission (FIG. 1) and are arranged ina straight line near the selector knob 15.

Each label P, R, N and D can be illuminated, e.g. by a LED (lightemitting diode) 18 located underneath the upper surface of the console14, each LED 18 being switched on or off according to the position ofthe illuminated pointer 16 as it will be described in further detail.

The illuminated pointer 16 is constituted by one 19 of several lightsources 20, for example LEDs, circumferentially distributed behind thetop surface 21 of the selector knob 15, which is illuminated when thevehicle is to be driven, e.g. when the driver turns the engine ignitionon. The arrangement of labels PRND and illuminated pointer 16 isconveniently referred to as a dynamic indicator for reasons which willbe explained. In a preferred arrangement, the surface of the selectorknob 15 includes a metallic centre piece 22 and a frosted transparentplastics ring 23 so that the LEDs 20 cannot be seen if not illuminated.

As illustrated in greater detail in FIGS. 2 to 8, the selector mechanismfurther includes, between the selector knob 15 and the base 13, a camplate or detent plate 24 which is rotatable around the shaft 17. Thedetent plate 24 forms part of a rotary selector 9 which is normallyconnected to the selector knob 15.

The detent plate 24 is connected to a sensor means in order to detect aposition of the selector knob 15 and to transmit an input to atransmission control unit 26 (FIG. 8). This sensor means includes asensor or encoder 25 that reads scan marks on a disc 27 fixed to thedetent plate 24 to provide a sensor output signal which isrepresentative of the direction and magnitude of the motion of thedetent plate 24.

The detent plate 24 is provided on its periphery with notches 29 over asector 28 and these can be engaged successively by a cam detent pin 30when the detent plate 24 rotates, as illustrated in FIG. 6 and FIG. 7.These notches 29 are arranged in such a way that each selection mode P,R, N, D and optional a manual mode (M) corresponds to an angularposition of the detent plate 24. Thus the cooperation between the detentpin 30 and the notches 29 forms an indexing means ensuring theimmobilization of the selector knob 15 in its different positions,respectively P, R, N and D. Furthermore, the depth and the shape of eachnotch are different for each position of the selector knob 15 so thatnot only is the driver able to feel the transition from one position toanother but can also infer the mode selected by the feel of the detent.

The detent plate 24 is provided on its periphery with a slot 32 whichextends over a sector 31 and this cooperates with a pin 33 which acts asa fixed end stop for the detent plate 24 when the pin 33 abuts eitherend of the slot 32.

In normal operation of the selector mechanism (FIG. 4), the selectorknob 15 and the detent plate 24 are connected to one another by means ofa first clutch device. The first clutch device comprises anelectromagnet clamp 34 vertically fixed on the detent plate 24 in frontof a first steel ring 35 fixed to the selector knob 15. Theelectro-magnet clamp is controlled by the transmission control unit 26in order to engage or disengage the detent plate 24 with the selectorknob 15 as it will be described further.

The selector knob 15 includes a locking device controlled by thetransmission control unit 26 for preventing selector knob movement undercertain fault conditions or when the selector knob 15 is in Parkposition and a brake pedal 37 has not been depressed. The locking deviceis, in the example shown, a second electromagnet clamp 36 arranged inthe same way as the first electromagnet clamp 34, the secondelectro-magnet clamp 36 being vertically fixed on the base 13 in frontof a second steel ring 38 fixed to the selector knob 15.

The selector mechanism further includes a powered selector correctionmode device or drive correction means 39 for rotating the detent plate24 in cases where an inappropriate selector mode position (PRND) hasbeen selected by the driver as it will be described in great detailbelow (FIG. 5). The drive correction means 39 comprises a bevel gearwheel 40 rotatable by means of an electric motor 41 and a bevel pinion42 under the control of the transmission control unit 26. The wheel 40includes a second clutch device arranged in the same way as the firstclutch device, i.e. comprising a third electro-magnet clamp 43 fixed onthe wheel 40 in front of a third steel ring 44 fixed to the detent plate24.

The transmission control unit 26 includes a known processing logiccircuit which is utilized for effecting automatic gearshifts within thetransmission in a known manner.

As show in FIG. 8, the transmission control unit 26 is connected to aCAN bus 7 which allows it to communicate with other control unitslocated in the vehicle, e.g. an engine control unit. In particular, eachsystem makes available on the CAN bus data from the sensors associatedwith it, data relating to its own operation and data relating to thevehicle's operation which it has derived from the information availableto it. Therefore the transmission control unit 26 receives over the CANbus the signals from wheel speed sensors and the brake pedal 37 but alsofrom a sensor connected to an emergency park release cable (not shown)as is usually provided on vehicle transmissions operated byshift-by-wire so as to shift the automatic transmission into Neutral toallow the car to be towed away.

Conventionally the transmission control unit 26 is also connected to adriver display 45 in order to indicate the currently engaged gear of thetransmission and possibly any transmission fault.

Operation of the transmission control unit and the selector mechanism isas follows.

Starting the engine causes one LED 19 of the illuminated pointer 16 onthe top of the selector knob 15 and the corresponding illuminated label18, i.e. normally P position, to turn on while stopping the enginescauses of the illuminated pointer 16 and the illuminated label 18 toturn off, hence avoiding any indication of the gear mode selected.

Thus, from a Park position of the selector knob, when the driverdepresses the brake pedal 37, a brake detection signal is sent over theCAN bus and is received by the transmission control unit 26 which thenunlocks the selector knob 15 by de-energising the second electromagnetclamp 36 and energises the first electromagnet clamp 34 to connect thedetent plate 24 with the selector knob 15. The driver can then select anappropriate transmission mode (FIG. 4). When the new position of thedetent plate 24 is detected by the encoder 25, the transmission controlunit 26 checks the availability of the request and shifts thetransmission by sending a command to an actuator 8 and turning on theillumination of the corresponding label.

If the transmission control unit 26 rejects the request (for example, ifReverse is selected when the vehicle is moving forward) then it sends asignal to the first electro-magnet clamp 34 to disengaged the detentplate 24 from the selector knob 15 by de-energising the firstelectromagnet clamp 34 and at the same time sends a signal to the secondand the third electromagnet clamp 36, 43 in order to connect the bevelwheel 40 of the drive correction means 39 to the detent plate 24 andlock the selector knob 15 with the base 13 (FIG. 5). The transmissioncontrol unit 26 then sends a command to the electric motor 41 to drivethe bevel pinion 42 and return the detent plate 24 to the previous mode.The transmission control unit 26 turns off both the actual illuminatedLED 19 of the dynamic pointer 16 and the illuminate label 18 selected bythe driver and turns on both the illuminated label 18 of the previousmode and the appropriate LED 40. The second and the third electro-magnetclamps 36, 43 can then be disengaged while the first electromagnet clamp34 is engaged so that the rotatable selector mechanism can again beoperated by the driver.

When the selector knob 15 comprises a manual mode position M the gearratios can be controlled in a pulse manner to select the next highest(+) or lowest (−) forward gear ratio by an auxiliary selector, e.g. by apair of paddles located near the steering wheel. The transmissioncontrol unit 26 may then send a command to the motor 41 and the thirdelectromagnet clamp 43 to rotate the detent plate 24 without moving theselector knob 15 from a Drive mode position to a Manual mode positionbut only changing the LED illumination 20 of the illuminated pointer 16and the corresponding label 18 when the driver starts to use theauxiliary selector. Similarly, the transmission control unit 26 may senda command to motor 41 and the third electromagnet clamp 43 to rotate thedetent plate 24 without moving the selector knob 15 from the Manual modeposition to the Drive mode position but only changing the LEDillumination 20 of the illuminated pointer 16 and the correspondinglabel 18 when the auxiliary selector is not used for a predeterminedtime period.

Under a fault condition of the transmission, the transmission controlunit 26 will send a command to lock the selector knob 15 in its currentposition and send a message to the display to alert the driver that thetransmission is in failure mode. In a preferred arrangement, all theelectro-magnet clamps 34, 36 and 43 will be engaged.

This method of controlling the selector knob 15 with a dynamic indicatorand a drive correction means 39 enables several advantages. Firstly, itmakes it possible to comply with legal requirements in case of powerfailure or when the park release is operated with the engine off when noindication of the current transmission mode will be indicated by thedynamic indicator. Secondly, in the situation where the driver moves theselector knob to an unavailable mode, the dynamic indicator and therotary selector 9 will be moved to indicate the actual transmissionmode. In addition, the selector knob 15 will not move but only thedetent plate 24 and the dynamic indicator, thus avoiding any attempts bythe driver to stop or hold the selector knob 15 in order to prevent thecorrection by the transmission control unit 26. Furthermore, thearrangement of a detent plate 24 which co-operates with the detent pin30 enables the provision of a shift-by-wire selector which hasmulti-stable positions giving, from the driver's viewpoint, a selectormechanism close to the conventional automatic transmission selectorlever having for each position mode of the selector a mechanical detentwith a configurable force profile.

Although the invention has been described by way of example withreference to a specific embodiment of the invention variousmodifications or additions may be made without departing from theinvention.

For example, it will be appreciated that under a fault condition or‘limp home’, the driver can still move the selector knob 15. This canlead the driver to think that a new selection mode, for instanceReverse, is available. Such operation of the selector knob 15 can leadto unexpected and possibly dangerous motion of the vehicle. Hence toovercome this problem, in addition the selector mechanism 10 may includean actuator, conveniently referred to as the retractor actuator (notshown), e.g. an electric motor, controlled by the transmission unitcontrol 26 that enables to retract the selector knob 15 inside a housing7 forming part of a centre console 3 mounted between the driver's seat 1and a passenger seat 4 into an inactive position, as show FIG. 9, whereit cannot be readily moved by the driver. Thus, the driver canphysically understand that the vehicle is in a failure mode and isdiscouraged from driving the vehicle for long periods in a faultycondition.

The retractor actuator may also be controlled by the transmissioncontrol unit 26 when the engine 12 is started or stopped. Starting theengine causes the selector knob 15 to rise from the housing 7 (FIG. 9)while stopping the engine causes the selector knob 15 to drop into thehousing 7 (FIG. 10). Thus, the selector knob 15 can be displaced eitherto an inactive position in which it is retracted inside the housing 7when the driver turns the engine ignition off or an active position inwhich it is projects outside the housing 7 when the driver turns theengine ignition on.

In a modification, the illuminated pointer could be replaced by amechanical pointer which can retract into a hidden position when theengine ignition is turned off or extend into a visible position when theengine ignition is turned on. The retractable pointer may be locatedinside or outside the selector knob. Also the electro-magnet clampdescribed above could be replaced by mechanical devices operating thesame function. Alternatively, the dynamic indicator could simplycomprise illuminated labels, the label corresponding to the current modeof the transmission being brighter than the others. Also, the selectorknob could be replaced by a lever moving about a horizontal axis, e.g.in a fore and aft plane. Furthermore, the selector mechanism couldinclude other functions mounted in the same housing, e.g. an enginestart button or a driving mode selector (casual driving, highwaydriving, driving on snow, etc.).

1. A selector mechanism for a motor vehicle transmission having a numberof operating modes, the selector mechanism having a housing, a rotaryhandle movable in the housing between a number of selector positions,each position being associated with a respective one of the operatingmodes of the transmission, a rotary selector which in normal use of theselector mechanism is connected to the rotary handle so as to rotatewith the rotary handle, sensor means for sensing the position of therotary selector and generating selector signals to a transmissioncontrol unit which in use supplies control signals to the transmission,an indexing means for maintaining the rotary handle in each of itspositions, a drive correction means and clutch devices to engage therotary selector to one of the rotary handle and the drive correctionmeans, wherein the clutch devices and the drive correction means in useare controlled by the transmission control unit to move the rotaryselector from a selector position selected by the driver by means of therotary handle to another selector position representative of theoperating mode of the transmission by means of the drive correctionmeans without moving the rotary handle.
 2. The selector mechanism ofclaim 1 and further comprising a dynamic indicator which in use isarranged to indicate the operating mode of the transmission.
 3. Theselector mechanism of claim 2 wherein the dynamic indicator in use isconnected to the transmission control unit to be visible when thevehicle is to be driven and move with the rotary selector.
 4. Theselector mechanism of claim 2 wherein the dynamic indicator comprises anilluminated pointer and an arrangement of illuminated labels indicatingthe selector mode.
 5. The selector mechanism of claim 4 wherein therotary handle has a top surface and the illuminated pointer isconstituted by a one of several light sources circumferentiallydistributed behind the top surface of the rotary handle.
 6. The selectormechanism of claim 1 and further comprising a retractor actuator whichin use is controlled by the transmission control unit to displace therotary handle between an active position where it can be manually movedbetween the selector positions by the driver of the vehicle, and aninactive position in which it is retracted inside the housing where itcannot be readily moved by the driver.
 7. The selector mechanism ofclaim 1 and further comprising a spring-loaded detent member carried bythe housing and wherein the rotary selector comprises a detent platehaving notches for cooperation with a spring-loaded detent member. 8.The selector mechanism of claim 7 and further comprising a stop carriedby the housing and wherein the detent plate has a slot which cooperateswith the stop to limit the rotation of the detent plate between the twoextreme selectable modes of the transmission.
 9. The selector mechanismof claim 7 wherein the sensor means for sensing the position of theselector comprises an encoder.
 10. The selector mechanism of claim 1 andfurther comprising a locking device which in use is controlled by thetransmission control unit to prevent rotation of the rotary handle whendisconnected from the rotary selector.
 11. The selector mechanism ofclaim 1 wherein one of the selector positions is a manual mode positionin which position the selector mechanism is operable for use with anauxiliary selector in order to control the gear ratios of thetransmission in a pulse manner.
 12. A selector mechanism for a vehicletransmission driven by an engine, the said selector mechanism having ahousing, a selector movable in the housing between a number of selectorpositions each associated with a respective operating mode of thetransmission, sensor means for sensing the position of the selector andgenerating selector signals to a transmission control unit which in usesupplies control signals to the transmission, and a retractor actuatorwhich in use is controlled by the transmission control unit to displacethe selector between an active position where it can be manually movedbetween the selector position by the driver of the vehicle, and aninactive position in which it is retracted inside the housing where itcannot be readily moved by the driver.
 13. The selector mechanism ofclaim 12 wherein the selector is a rotary switch.
 14. The selectormechanism of claim 13 and further comprising a drive correction meanswhich in use is connected to the transmission control unit to move theselector from a selector position selected by the driver to anotherselector position representative of the operating mode of thetransmission.
 15. The selector mechanism of claim 14 wherein the drivecorrection means is operable to prevent manual movement of the selector.16. A method of controlling a selector mechanism of a motor vehiclehaving a transmission, a transmission control unit, at least one sensorfor sensing a position of a rotary selector on the selector mechanism,an index mechanism for maintaining a rotary handle of the selectormechanism in one of a predetermined number of selector positionscorresponding to one of a predetermined number of transmission operatingmodes, and drive correction means, and a clutch mechanism engaging therotary selector to one of the rotary handle and drive correction means,wherein the clutch mechanism and the drive correction means arecontrolled by the transmission control unit to move the rotary selectorfrom a selector position selected by the driver through the rotaryhandle to another selector position representative of the operating modeof the transmission by means of the drive correction means withoutmoving the rotary handle, the method comprising the steps of:determining an operational state of the vehicle and providing datarelated to said operational state to the transmission control unit;receiving a signal from the sensors indicating whether a selector modeis available or unavailable; if the selector mode is unavailable thetransmission control unit operates the clutch mechanism to engage therotary selector to the drive correction means and disengage the rotaryselector from the rotary handle; and sending a command to the drivecorrection means to return the rotary selector to the previous gearposition.
 17. The method of claim 16 wherein the selector mechanismfurther comprises a dynamic indicator which in use is arranged toindicate the operating mode of the transmission, the method includingthe further step of sending a command from the transmission control unitto the dynamic indicator to indicate the mode of the transmissionaccording to the motion of the rotary selector.
 18. The method of claim17 wherein the dynamic indicator of the selector mechanism furthercomprises an illuminated pointer and an arrangement of illuminatedlabels indicating the selector mode, the method including the furtherstep of the transmission control unit sending a command to turn theilluminated pointer off and a command to illuminate the label of thecurrent selector mode.
 19. The method of claim 16 when the selectormechanism further comprises a retractor actuator controlled by thetransmission control unit to displace the rotary handle between anactive position where it can be manually moved between the selectorpositions by the driver of the vehicle, and an inactive position inwhich it is retracted inside the housing where it cannot be readilymoved by the driver, the method further including the step of sending acommand from the transmission control unit to the retractor actuator tomove the rotary switch to the active position when the engine is runningand to move the rotary switch to the inactive position when the engineis stopped.
 20. The method of claim 19 further including the step ofupon the transmission control unit receiving a failure signal, thetransmission control unit sending a command to the retractor actuator tomove the selector towards the inactive position from the activeposition.